The present invention relates to an optical scanner and, more particularly, to a laser raster output scanner (ROS) particularly suitable for use in a compact system design.
Printing systems utilizing lasers to reproduce information are well known in the art. Typically, such systems include a laser for generating a laser beam, a modulator, such as an acousto-optic modulator, for modulating the laser beam in accordance with input information to be reproduced, a multifaceted polygon scanner for scanning the modulated laser beam across a medium on a line to line basis, and various optical components to focus and align the laser beam onto the medium.
In some printing systems it is required that a cylindrical lens be interposed between the modulated laser beam and the scanner to provide a fan fold light output beam which fully illuminates at least one facet of the scanner. The cylindrical lens is preferably aligned with the laser beam to redistribute the energy of the modulated laser beam in a manner whereby the energy distribution of the laser beam incident on the recording medium is substantially symmetrical about the start of scan and end of scan positions of the medium, best focus ideally occurring at the center of the scan line. This is particularly important in those systems which utilize a xerographic recording medium.
A compact design for the scanning optics of these prior art type of printers is desirable to make the machine itself as compact as possible. The scanning system is usually reduced in total area by folding the beam path by the use of a plurality of mirrors. Examples of a compact modular laser printing system are the Xerox 4045 printer and the system shown in U.S. Pat. No. 4,230,902.
It would be desirable to improve the efficiency of these prior art systems by using as few optical elements as possible to decrease hardware costs and assembly and alignment costs. The present invention is directed toward replacement of two elements in the pre-polygon prior art optics, a reflector used to create a folding path, and a cylinder lens. These two elements are to be replaced by a single component, a cylindrical mirror element. This replacement, besides leading to manufacturing economies also provides a more optimum, slow-scan focus at the polygon facets because of the properties of the cylindrical mirror. More particularly the invention relates to a raster output scanning system comprising:
a light source for generating a coherent collimated light beam output along an optical path,
a photosensitive image plane,
a rotatable multifaceted polygon interposed in the optical path between the light source and the photosensitive image plane for scanning light beams directed onto the facets in a fast scan direction across the photosensitive image plane,
a post polygon optical system to focus the reflected lines from the polygon in the fast scan direction, and
a pre-polygon optical system including a positive cylindrical mirror positioned in the optical path between the light source and the polygon, cylindrical mirror adapted to change the direction of the optical path through its reflective properties, and to focus the input light to a beam line which has a vertical plane therethrough coincident with a vertical plane through the vertical surface of the facets.